Conventional guns and projectile launching weapons utilize the burning of chemical propellants to achieve high projectile velocities. In recent years, there has been a renewed interest in projectile launchers which utilize electromagnetic energy. Such electromagnetic launchers may find application in space launch weaponry and impact fusion as well as in more conventional ordnance. Generally speaking, electromagnetic launchers promise greater projectile velocities than launchers utilizing chemical propellants.
In electromagnetic launchers (also called railgun) large current pulses are introduced into current-carrying rail conductors to accelerate a projectile (often termed an armature).
An example of a novel railgun design together with a discussion of its operational principles and prior art, is contained in applicant's co-pending application entitled "Electromagnetic Injector/Railgun," Ser. No. 910,915, now abandoned, filed Sept. 22, 1986, the entire disclosure of which is hereby incorporated by reference.
In general, in railgun applications, armature velocity increases with increasing current. However, the magnitude cannot be increased without limit due to joule heating of the rails, together with radiative heating of the railgun materials by plasmas, and the structural loading on the rails created by high magnetic pressures. Joule heating of the rails is most severe when a fast-moving armature exposes new conductor material to intense currents which do not have time to diffuse into the body of the conductor. The joule heating effect predominates at the armature-rail interface. The joule heating effect causes severe rail erosion.
The rail erosion problem is perhaps the most serious technical problem which must be overcome before railguns can compete with conventional chemical propellant guns. Short rail operating life and low firing rate are detrimental characteristics of todays railguns.
Recent developments in the field of superconductivity have produced a variety of new ceramic-type materials which are capable of achieving the superconducting state at critical temperatures above 77.degree. K., the boiling point of liquid nitrogen. The critical temperature is the temperature at which the material becomes superconducting. The new class of materials (termed for convenience "superconducting ceramics" herein--even for materials which are not basically ceramic in nature) have been extensively discussed in the popular press. For example, the New York Times, on Mar. 20, 1987, reported the existence of superconducting ceramics and described the making of such materials into sheets of vinyl-like tapes and washer shapes. Furthermore, Electronics in its Apr. 2, 1987 issue on pp 49-51 reported the making of superconducting ceramics into wire shapes.
The composition and manufacture of superconducting ceramics is discussed, for example, in Physics Today pp 17-23, April 1987 which is incorporated herein by reference. An entire class of compounds with the chemical composition are RBa.sub.2 Cu.sub.3 O.sub.9-y, where R stands for a transition material or a rare earth ion and Y is a number less than 9, preferably 2.1.+-.0.05 has demonstrated superconductive properties above 90.degree. K. This class of materials is included in the terms "superconducting ceramic" and "rare earth doped copper oxide" as used herein. Scandium, lanthanum, neodymium, samarium, europium, gadolinium, dysprosium, holmium, erbium, ytterbium, yttrium, and lutetium are acceptable substitutes for R above. The crystal structure of these compounds is described as an orthorhombically distorted perovskite structure.
Fabrication of superconducting ceramics is discussed in the above-mentioned Physics Today article. A detailed discussion of the fabrication and physical properties of a typical superconducting ceramic is also found in "R. J. Cava et al.," Bulk Superconductivity at 91.degree. K. in Single Phase Oxygen-Deficient Perovskite Ba.sub.2 YCu.sub.3 O.sub.9-8, Physical Review letters, pp 1676-1679, 20 April Vol. 58, No. 16.
Superconducting ceramics with high critical current densities (in excess of 10.sup.5 A/cm.sup.2) may be produced by growing epitaxial films of RBa.sub.2 Cu.sub.3 O.sub.7-x on SrTiO.sub.3 substrates as taught in P. Chaudhari et al., "Critical Current Measurements in Epitaxial Films of YBa.sub.2 Cu.sub.3 O.sub.7-x Compound," Phys. Rev. Ltrs. vol. 58, No. 25, pp 2684-2686, June 22, 1987.
Another recently discovered high temperature superconductor is: BiBaCaCu.sub.2 O.sub.4. Is has been reported that the aforementioned compound becomes superconducting at temperatures over 110.degree. K.
The attractiveness of high temperature superconductors is enhanced by the advent of miniature open cycle Joule-Thomson coolers. In a Joule-Thomson cooler, a fluid at high pressure is allowed to flow through a flow restrictor into a region of low pressure where it expands and cools in the process. Usually, the cooling fluid is high pressure argon or nitrogen. The new Joule-Thomson coolers are very compact, being 0.4" diameter and 2" long. Approximately 6 seconds is required to cool a 2-4 gram mass of high temperature superconductor from room temperature to below 80.degree. K. The weight of the gas bottle is approximately 10 lbs. It is expected that a typical Joule-Thomson cooler is capable of cooling approximately 100 small armatures.
Those concerned with the development of electromagnetic launchers have engaged in a continual search to eliminate the problems of arcing and attendant rail erosion. The new superconducting ceramics offer potential solutions to these problems.